1. Field of the Invention
The present invention relates generally to biocidal compositions, and more particularly, compositions comprising composite particles, wherein each of the composite particles contains a shell and a core possessing differing biocidal activity profiles. The core comprises a biocidally active oxidized metal, or metal-containing compound, and the shell comprises a pyrithione salt of the core metal having complimentary biocidal activity to the activity of the core. Select compositions of the present invention, when incorporated into a matrix, such as a paint or other coating composition, provide the matrix with resistance to fouling by organisms of more than one type, thus providing broad spectrum biocidal activity.
2. Description of the Related Art
Certain composite particles containing a biocide are known. By way of illustration, U.S. Pat. No. 5,510,109 discloses an antibacterial/antifungal material carried on a porous particle carrier that is preferably a silica gel particle. As another illustration, U.S. Pat. No. 5,595,750 discloses an antimicrobial composition comprising an inorganic particle having a first coating possessing antimicrobial properties and a second coating providing a protective function for the composition. Heretofore, however, no composite biocide particles have been known, based upon the knowledge of the present inventors, comprising two biocides with differing biocidal activity profiles.
In the marketplace, biocides enjoy wide usage. For example, biocidal compositions are employed in a wide variety of applications, including shampoos, soaps, skin care medicaments, as well as paints, particularly marine paints. Fouling of marine equipment has been a persistent problem for many years. For example, boat hulls, docks, buildings, fishnets and cages, and related marine equipment that are in constant contact with marine water become fouled with algae, barnacles and other marine organisms, and much time and expense is required to remove these organisms.
Generally, marine fouling has been categorized into two broad classes. “Soft-fouling” refers to growth of “soft” organisms, such as algae on marine and coastal equipment and other outdoor structures. “Soft” organisms also include the fungi causing mildew, often co-existing with algae, and typically necessitating the addition of both an algaecide and a fungicide in outdoor coatings such as architectural paints. “Hard-fouling”, by contrast, refers to growth of “hard” organisms such as barnacles, and tubeworms. Compounds used to control hard fouling include copper, cuprous oxide, zinc oxide, and copper thiocyanate. As used herein “anti-soft-fouling” refers to an antifouling agent that is effective in reducing or preventing the growth of soft organisms, whereas “anti-hard-fouling” refers to an antifouling agent that is effective in reducing or preventing the growth of hard organisms. Due to the fact that there exist different organisms that cause soft-fouling versus hard-fouling, separate approaches have been taken to address each type of fouling.
Polyvalent metal salts of pyrithione (also known as 1-hydroxy-2-pyridinethione; 2-pyridinethiol-1-oxide; 2-pyridinethione; 2-mercaptopyridine-N-oxide; and pyridinethione-N-oxide) have gained acceptance for use in marine paints and coatings to decrease or minimize soft-fouling. These salts are known to be effective biocidal agents, leading to their wide usage as algaecides, fungicides and bactericides in paints and personal care products such as anti-dandruff shampoos. Generally these salts are only sparingly soluble in water, as typified by magnesium pyrithione, barium pyrithione, bismuth pyrithione, ferric pyrithione, strontium pyrithione, copper pyrithione, zinc pyrithione, cadmium pyrithione, and zirconium pyrithione. The most widely used divalent pyrithione salts are zinc pyrithione and copper pyrithione. Synthesis of polyvalent pyrithione salts is described in U.S. Pat. No. 2,809,971 incorporated herein by reference in its entirety. Other patents disclosing similar compounds and processes for making them include U.S. Pat. Nos. 2,786,847; 3,589,999; 3,590,035; and 3,773,770, each of which is incorporated herein by reference in their entireties.
Zinc pyrithione is useful as an antimicrobial agent active against gram-positive, some negative bacteria, fungi, algae and yeasts. Suspensions of zinc pyrithione are also used as antibacterial, antifungal, and antialgael additives to provide soft-fouling protection for paints and other coating compositions. However, zinc pyrithione is not as effective against hard-fouling organisms as might be desired.
Copper pyrithione is sold commercially as an algaecide and anti-soft-fouling agent for marine paints and coatings as disclosed in U.S. Pat. Nos. 5,246,489 and 5,540,860, both of which are incorporated by reference in their entireties. Copper pyrithione offers several advantages over zinc pyrithione for many applications, notably a lower solubility (about 1 ppm versus 6-10 ppm for zinc pyrithione). The lower solubility of copper pyrithione, as compared to zinc pyrithione, increases its effective availability as a biocidal agent over a longer period of time when exposed to marine environments, and makes it particularly desirable as an anti-soft-fouling agent. However, copper pyrithione is not as effective in preventing hard-fouling as might be desired. To address this shortcoming, combinations of soft-fouling and hard-fouling agents have been manufactured. However, to date, these combinations have not been entirely successful.
Combinations of zinc pyrithione or copper pyrithione and copper compounds such as cuprous oxide, copper hydroxide, or copper thiocyanate are known to be useful antifouling agents when formulated into marine paint compositions to control both hard- and soft-fouling. Such a paint composition is disclosed in U.S. Pat. No. 5,057,153, herein incorporated by reference in its entirety. However, paint compositions which include zinc pyrithione and cuprous oxide antifouling agents tend to form undesirable gels during storage, thereby limiting the useful life of the composition. The gelation problem, and efforts to resolve or mitigate it are described in U.S. Pat. Nos. 5,298,061 and 5,342,437.
Tributyl tin compounds have in the past been demonstrated to be effective in controlling soft-fouling, and to a lesser degree, hard-fouling, organisms. However, tributyl tin compounds are toxic to non-target organisms, and are persistent, thus posing an unwanted environmental hazard.
In addition, these toxic compounds persist in the environment after application and continue to exert their toxic effects for years. Therefore, use of tributyl tin compounds as antihard- and antisoft-fouling agents is restricted in many parts of the world. Accordingly, what the paints and coatings community needs is a safer antimicrobial agent that is effective in controlling both soft- and hard-fouling growth simultaneously. In this regard, zinc and copper compounds, such as zinc oxide, cuprous oxide, and pyrithione salts, such as zinc pyrithione or copper pyrithione, have been individually added to antifouling paints. However, such a combination made by separately adding these two components to a paint has been found to provide less performance efficacy (based upon the amounts of individual components added to the paint) than might otherwise be desired. Accordingly, new methodology for providing relatively environmentally safe combinations of anti-hard and anti-soft components within a single biocidal composition package for use in paints and other coatings would be highly desired by the coatings manufacturing community. The present invention provides one answer to that need.